One of the primary components in paint is the "film former" that provides a film for the protective function for a substrate coated with the paint. Film forming components of liquid paint include resins which have required organic solvents to provide the resins with suitable viscosities such that the paint can be applied by existing commercial application equipment. Use of organic solvents, however, raises at least two problems. In the past and potentially in the future, petrochemical shortages mitigate against the use of organic solvent in great volumes. Second, environmental concern mitigates against the use of organic solvents.
Environmental concern has become increasingly important. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to volatile organic compounds (VOCs) which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and industrial coating industry which uses organic solvents which are emitted into the atmosphere.
To reduce organic solvent content and VOCs, researchers have developed high solids coating compositions and powdered coating compositions. High solids compositions generally are liquid and are designed to minimize solvents. Powdered coating compositions are solid powders and generally eliminate solvents. While each have advantages, each coating composition has disadvantages.
Coating compositions which include high solids polymeric vehicles based upon polyesters have become popular. In high solid polyesters as opposed to "conventional" compositions which use organic solvents, high molecular weight generally needs to be achieved during crosslinking rather than being attained from the basic polyester polymer. Hence, high solids polyesters normally supply a greater number of reactive sites (predominantly hydroxyl groups) available for crosslinking. The resultant polymers typically exhibit 70-80% solids-weight when reacted stoichiometrically with isocyanate crosslinkers, but frequently yield empirical solids up to 12% lower, when crosslinked with melamine resins. Despite their reduced use of organic solvents, high solids polyester coating compositions could be produced on the same equipment and be employed in many of the same applications as lower solids "conventional" polyester coating compositions. Further, as a result of their many strengths such as ease of manufacturing and use, low volatile emissions, reduced energy requirements, greater application efficiency, lower handling and storage costs, and excellent physical properties, high solids polyester coating compositions have enjoyed spectacular growth in manufacture and use. They still require organic solvents, however, and are a source of VOCs.
Powder coatings and UV-curable coatings are desirable ultrahigh or 100% solids coatings. However, there are limitations as to the techniques and the equipment which is used to apply the powdered and UV-curable compositions.
To reduce solvent content and VOCs in polymeric vehicles and formulated coating compositions for paints, researchers have been driven by three major objectives: controlling the reactivity of the film forming components in the paint; keeping the viscosity of the components in the paint low to minimize the organic solvents in the paint and to keep the VOCs in the paint at the lowest possible level; and keeping the components in the paint at a low volatility to minimize VOCs.
High viscosity is a major problem which needs to be solved in ultrahigh or 100% solids coatings. In high solids polyester coatings, the viscosity of concentrated polyester solutions depends on several variables. Molecular weight and molecular weight distribution are two important factors. According to polymer physics theory, the viscosity of polymers in the liquid state depends mainly on the average molecular weight and the temperature, so it is desirable to reduce average molecular weight for solventless polyester coating. The major factor controlling molecular weight (M.sub.n) of a polyester is the mole ratio of dibasic acid/diol or polyol. A dibasic acid to diol or polyol ratio of the order of 2:3 is typical. However, loss of polyol during the production of the polyester can result in a significantly higher molecular weight than predicted from the starting ratio. It is necessary to add some extra glycol to compensate for loss. In very high solids coatings, the low molecular weight fraction of resin may be volatile enough to evaporate when a thin film is baked. Such loss has to be counted as part of the VOC emissions.
The number of functional groups per molecule also affects the viscosity because of hydrogen bonding. Most oligomers or polymers require high functionality to achieve a highly crosslinked film and reasonable Tgs to have adequate film properties for most applications. The high functionality tends to increase the viscosity significantly.
An object of the invention is to provide a polymeric vehicle which will reduce or eliminate VOCs in coating compositions by providing a polymeric vehicle which is effective for providing a formulated coating composition which does not require organic solvent to reduce the viscosity of the formulated coating composition for application of the formulated coating composition.
Another object of this invention is to provide polymeric vehicles which are not only low in VOCs and effective in providing solventless formulated coating compositions, but which provide coating binders with good film properties such as hardness and impact resistance.
Yet another object of this invention is to control the viscosity to low levels at a specific application shear rate of a liquid polymeric vehicle or a liquid formulated coating composition without using organic solvents or water for such control.
Further, objects and advantages of the invention will be found by reference to the following description.